The invention relates generally to the field of plasma arc torches and systems. In particular, the invention relates to an electrode for use in a plasma arc torch having an enhanced cooling configuration.
Plasma arc torches are widely used in the processing (e.g., cutting and marking) of metallic materials. A plasma arch torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. The gas can be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air.
In process of plasma arc cutting or marking a metallic workpiece, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). The pilot arc ionizes gas passing through the nozzle exit orifice. After the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc then transfers from the nozzle to the workpiece. The torch is operated in this transferred plasma arc mode, characterized by the conductive flow of ionized gas from the electrode to the workpiece, for the cutting or marking the workpiece.
U.S. Pat. No. 4,902,871, assigned to Hyperthemi, Inc. describes and claims an apparatus and method for cooling a xe2x80x9cspiral groovexe2x80x9d electrode in a contact start torch. A gas flow passage, preferably a spiral fin machined on the outer side surface of the shoulder portion, diverts a portion of the gas flow from the plasma chamber to a region above the electrode where it is vented to atmosphere. The fin is machined to form a spiral groove that is sufficiently constricted that a substantial pressure drop appears along the path, while allowing a sufficient gas flow to produce the desired cooling. The adjacent portions of the spiral fin are preferably closely spaced to enhance the surface area of the electrode in a heat transfer relationship with the cooling gas flow.
While spiral groove electrodes operate as intended, applicants have perceived the need for an alternative form of the electrode which is simpler to manufacture, but still provides the same benefits as the spiral groove electrode.
The present invention resides in the recognition that an electrode having a ribbed configuration is easy to manufacture and provides a large surface area for cooling the electrode. The ribbed configuration provides for a plurality of independent cooling passages that extend from a first (front) end to a second (aft) end of the electrode. In one embodiment, the electrode includes an elongated electrode body having a first end and a second end. The electrode also includes a shoulder having an enlarged diameter body integral with the electrode body. The shoulder has an imperforate face toward the first end and at least one rib extending aft of the face towards the second end of the electrode body.
The at least one rib has a varying height forming at least one groove in the shoulder body of varying depth. In one embodiment, the depth of each groove is greater toward the second end of the electrode than toward the first end. The at least one rib has an orientation between limits of being longitudinally aligned and substantially circumferentially disposed relative to the electrode body. As stated previously, these grooves act as independent, parallel cooling passages that provide a large surface area and facilitate substantial cooling of the electrode.
In a detailed embodiment, the electrode can comprise a high thermal conductivity material (e.g., copper) and can have an insert disposed in a bore formed in at least one of the first end and the second end. The insert can comprise a high thermionic emissivity material (e.g., hafnium or zirconium), and the shoulder can have an enlarged body of constant diameter that includes a plurality of ribs (and grooves).
The present invention also features a method of cooling an electrode in a torch body of a plasma arc torch. The torch includes a nozzle disposed relative to the electrode and a swirl ring to define a plasma chamber. The electrode is provided comprising an elongated electrode body having a first end and a second end. The electrode also includes a shoulder having an enlarged diameter body integral with the electrode body. The shoulder has an imperforate face toward the first end and a plurality of ribs extending aft of the face toward the second end of the electrode. A flow of pressurized gas is directed to the plasma chamber via the swirl ring. A portion of the pressurized plasma gas is directed through the plurality of grooves between the ribs to a rear chamber. The grooves act as parallel, independent cooling paths to cool the electrode.